Electric gated integrator detection method and device thereof

ABSTRACT

A cavity ring down system is optimized to precisely measure trace gases or particles in an air sample by using time sampling detection and multiple-sample averaging resulting in a high signal-to-noise ratio. In one embodiment, a cavity ring down system is programmed to measure the rise time and the fall time of the light level in an optical cavity. The cavity ring down system is programmed to integrate a plurality of sample portions during a rise time and a plurality of sample portions during a fall time (in alternate intervals) to obtain a time constant with no sample present and a time constant with sample present. The measurements are used to calculate trace gases in the air sample.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present Utility Application for Patent claims priority to U.S.Provisional Application No. 61/050,115 entitled “Electric GatedIntegrator Detection & Device Thereof” filed May 2, 2008, and herebyexpressly incorporated by reference herein. May 2, 2009 falls on aSaturday, consequently the present application has been filed on Monday,May 4, 2009.

FIELD OF INVENTION

At least one feature pertains to the detection and measurement of tracechemical species or particles in gaseous samples.

BACKGROUND OF INVENTION

Air is a mixture of gases approximately composed of 78.08% nitrogen(N₂), 20.95% oxygen (O₂), 0.93% argon (Ar), 0.038% carbon dioxide (CO₂),trace amounts of other gases, and a variable amount (average around 1%)of water vapor. At ambient temperatures, the oxygen and nitrogen gasesin air will not react with each other. However, in an internalcombustion engine, combustion of a mixture of air and fuel producescombustion temperatures high enough to drive endothermic reactionsbetween atmospheric nitrogen and oxygen in the flame, yielding variousoxides of nitrogen, such as nitric oxide (NO) and nitrogen dioxide(NO₂). Mono-nitrogen oxides such as NO and NO₂ are typically referred toby the generic term NO_(x).

Nitrogen dioxide (NO₂) is a major pollutant in the atmosphere of moderncities that is easily recognized by its reddish brown color. NO₂ isformed when nitric oxide (NO) is produced as a byproduct of combustionin internal combustion engines and power generators at temperaturesgreater than 800° C. and is oxidized by alkyl peroxy radicals in theatmosphere. In California and much of the United States, a principalsource of NO is from trucks, since auto emissions have been successfullyreduced by use of catalytic converters. NO₂ in the tropospheresubsequently undergoes photolysis to ultimately form ozone (O₃) in thepresence of sunlight. In the stratosphere, however, NO₂ is implicated inthe destruction of O₃. Mixing ratios for NO₂ have been measured atsub-parts-per-billion levels in remote areas and up to hundreds of partsper billion (ppb) in urban areas.

Although techniques have been developed to measure atmospheric NO₂,these techniques have deficiencies related to interferences, stabilityand precision. As a result, the measured atmospheric NO₂ may not beaccurate. Consequently, a technique to measure atmospheric NO and NO₂that lacks interference and is more stable and precise is needed.

SUMMARY OF INVENTION

A method of measuring the presence of a gas in an air sample using acavity ring down system, comprising: (i) collecting at least one currentresulting from light introduced into an optical cavity when a lightsource is turned ON and while the air sample is being introduced intothe optical cavity; (ii) measuring the at least one current resultingfrom light introduced into the optical cavity when a light source isturned ON; (iii) collecting at least one current resulting from lightintroduced into the optical cavity when the light source is turned OFFand while the air sample is being introduced into the optical cavity;and (iv) measuring the at least one current resulting from lightintroduced into the optical cavity when a light source is turned OFF isherein disclosed

The method may further include: (i) collecting a plurality of additionalcurrents resulting from light introduced into the optical cavity when alight source is turned ON; (ii) summing the plurality of additionalcurrents resulting from light introduced into the optical cavity when alight source is turned ON to obtain a sample integrated rise time; (iii)collecting a plurality of additional currents resulting from lightintroduced into the optical cavity when a light source is turned OFF;and (iv) summing the plurality of additional currents resulting fromlight introduced into the optical cavity when a light source is turnedOFF to obtain a sample integrated fall time. The collecting may beperformed by a detector during a predetermined time period when thelight source is ON or OFF.

The method may further include: (i) collecting at least one currentresulting from light introduced into the optical cavity when a lightsource is ON and while no sample is being introduced into the opticalcavity; (ii) measuring the at least one current resulting from lightintroduced into the optical cavity when a light source is ON; (iii)collecting at least one current resulting from light introduced into theoptical cavity when the light source is OFF and while no sample is beingintroduced into an optical cavity; and (iv) measuring the at least onecurrent resulting from light introduced into the optical cavity when alight source is OFF. The method may further include: (i) collecting aplurality of additional currents resulting from light introduced intothe optical cavity when a light source is ON while no sample is beingintroduced; (ii) summing the plurality of additional currents resultingfrom light introduced into the optical cavity when a light source is ONwhile no sample is being introduced to obtain a reference integratedrise time; (iii) collecting a plurality of additional currents resultingfrom light introduced into the optical cavity when a light source is OFFwhile no sample is being introduced; and (iv) summing the plurality ofadditional currents resulting from light introduced into the opticalcavity when a light source is OFF while no sample is being introduced toobtain a reference integrated fall time. The collecting may be performedby a detector during a predetermined time period when the light sourceis ON or OFF. The plurality of currents may be collected between 10,000and 100,000 times. The method may further include using the referenceintegrated rise time, the reference integrated fall time, the sampleintegrated rise time and the sample integrated fall time to calculate anamount of absorbing gas present in the air sample wherein the amount ofabsorbing gas is represented by (i) the difference between referenceintegrated rise time and the sample integrated rise time and (ii) thedifference between the reference integrated fall time and the sampleintegrated fall time. The light source may be a light-emitting diode orlaser. The absorbing gas may be at least one of nitrogen dioxide,nitrogen trioxide, nitrous oxide, fluorine, chlorine, bromine, iodine,ozone, sulfur dioxide, chlorine dioxide, HO₂ radicals, OH radicals,formaldehyde, aldehydes, hydrocarbons, or an aromatic species. Thecombination of scattering and absorption by particles may be measured bythe cavity ring down system.

A method of measuring the presence of a gas in an air sample using acavity ring down system, including: when no sample is introduced intothe system: (i) measuring an output voltage from a plurality of currentsresulting from light introduced into an optical cavity when a lightsource is ON during a predetermined interval of a rise time of the lightwherein the resulting measurement is a reference rise time measurement;(ii) measuring an output voltage from a plurality of currents resultingfrom light introduced into the optical cavity when the light source isOFF during a predetermined interval of a fall time of the light whereinthe resulting measurement is a reference fall time measurement; (iii)measuring an output voltage from a plurality of currents resulting fromlight introduced into the optical cavity when a light source is ONduring the rise time of the light wherein the resulting measurement is areference steady state rise time measurement; and (iv) measuring anoutput voltage from a plurality of currents resulting from lightintroduced into the optical cavity when a light source is OFF during thefall time of the light wherein the resulting measurement is a referencesteady state fall time measurement is herein disclosed.

The method may further include: when an air sample is introduced intothe system: (i) measuring an output voltage from a plurality of currentsresulting from light introduced into an optical cavity when a lightsource is ON during a predetermined interval of a rise time of the lightwherein the resulting measurement is a sample rise time measurement;(ii) measuring an output voltage from a plurality of currents resultingfrom light introduced into the optical cavity when the light source isOFF during a predetermined interval of a fall time of the light whereinthe resulting measurement is a sample fall time measurement; (iii)measuring an output voltage from a plurality of currents resulting fromlight introduced into the optical cavity when a light source is ONduring the rise time of the light wherein the resulting measurement is asample steady state rise time measurement; and (iv) measuring an outputvoltage from a plurality of currents resulting from light introducedinto the optical cavity when a light source is OFF during the fall timeof the light wherein the resulting measurement is a sample steady statefall time measurement.

The reference rise time measurement, the reference fall timemeasurement, the reference steady state rise time measurement, thereference steady state fall time measurement, the sample rise timemeasurement, the sample fall time measurement, the sample steady staterise time measurement and the sample steady state fall time measurementmay be used to calculate the amount of gas in the air sample. Eachoutput voltage may be collected between 10,000 and 100,000 times. Thelight source may be a light-emitting diode or laser. The absorbing gasmay be at least one of nitrogen dioxide, nitrogen trioxide, nitrousoxide, fluorine, chlorine, bromine, iodine, ozone, sulfur dioxide,chlorine dioxide, HO₂ radicals, OH radicals, formaldehyde, aldehydes,hydrocarbons, or an aromatic species. The combination of absorption andscattering of particles may be measured by the cavity ring down system.

A method of measuring the presence of a gas in an air sample using acavity ring down system, including at least one of: (a) collecting atleast one current resulting from light introduced into an optical cavitywhen a light source is turned ON and while the air sample is beingintroduced into the optical cavity; and measuring the at least onecurrent resulting from light introduced into the optical cavity when alight source is turned ON; or (b) collecting at least one currentresulting from light introduced into the optical cavity when the lightsource is turned OFF and while the air sample is being introduced intothe optical cavity; and measuring the at least one current resultingfrom light introduced into the optical cavity when a light source isturned OFF is herein disclosed. The measurements resulting from step (a)or step (b) may be used in combination with a total signal intensity tocalculate the amount of gas in the air sample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of a cavity ring down system according toan embodiment of the invention.

FIG. 2 is a graphical representation of a “rise” time signal measurementgenerated from a CRDS during a predetermined time period.

FIG. 3 is a graphical representation of a “fall” time signal measurementgenerated from a CRDS during a predetermined time period.

FIG. 4 is a graphical representation of an example rise time measurementinterval.

FIG. 5 is a graphical representation of an example fall time measurementinterval.

DETAILED DESCRIPTION

The following detailed description is of the best currently contemplatedmodes of carrying out the invention. The description is not to be takenin a limiting sense, but is made merely for the purpose of illustratingthe general principles of the invention.

One commonly used method of measuring atmospheric nitrogen dioxide (NO₂)is chemiluminescence in which conversion of NO₂ to nitric oxide (NO),either by catalytic thermal decomposition (which suffers frominterferences from organic nitrates, HONO, HNO₃, etc.) or photolysis(which is relatively immune from interferences), is followed by reactionof NO with O₃ to produce electronically excited NO₂*. The excited NO₂*emits a broad continuum radiation in the region of 500-900 nanometers(nm), with a signal strength that is proportional to the concentrationof NO. Subtraction of the background NO concentration then yields theconcentration of NO₂.

Chemiluminescence of nitric oxide (NO) by reaction with ozone is usedextensively for quantifying NO and nitrogen dioxide (NO₂) in industrialsmoke stack emissions, air quality monitoring stations and medicalfacilities, but suffers from quenching by water vapor and, at highenough concentrations, from CO₂, as well leading to erroneously lowreadings. An additional problem for NO₂ measurements usingchemiluminescence is that catalytic thermal decomposition of NO₂ to NOfor detection together as NO_(x), where x=1 and/or 2, can lead to highNO₂ readings from other nitrogen-containing species, such as acylperoxynitrates (PANs), alkyl nitrates and ammonia which all produce NO₂upon thermal decomposition. This additional signal has resulted inNO_(x)-analyzers being termed NO_(y)-analyzers because they measure morethan the sum of NO and NO₂. In the presence of quenching, the analyzerscan actually indicate significantly less pollution as well. As a result,accurate measurements of NO₂ using the prior art approach ofchemiluminescence cannot be reliably obtained.

In addition to the prior art approach of chemiluminescence, several NO₂specific analyzers with low limits of detection have been demonstratedusing techniques including cavity ring-down spectroscopy (CRDS) and itsderivatives, continuous wave cavity ring-down (cw-CRDS), off-axiscw-CRDS, cavity attenuated phase shift spectroscopy (CAPS), and cavityenhanced absorption spectroscopy (CEAS). Tunable diode laserspectroscopy (TDLAS) and laser induced fluorescence (LIF) are moreestablished techniques that measure NO₂ and could also be combined withchemiluminescence.

CRDS is a sensitive spectroscopy technique that is based on measurementsof the rate of attenuation (k) rather than the magnitude of attenuationof the light by a sample. It can be used to measure the concentration ofsome light-absorbing substances, such as air pollutants. In CRDS, twoultra-high reflective mirrors face each other with a space (or cavity)in between. In the conventional pulsed laser implementation, a briefpulse of light is injected into the cavity and bounces (i.e., “rings”)back and forth between the mirrors. Some small amount (typically around0.1% or less) of the generated light enters and leaks out of the cavityand may be measured each time light hits one of the mirrors. As somelight is lost (i.e., leaks out) on each reflection, the amount of lighthitting the mirrors is slightly less each time. Furthermore, as apercentage leaks through, the amount of light measured also decreaseswith each reflection. If the only loss factor in the cavity is thereflectivity loss of the mirrors, one can show that the light intensityinside the cavity decays exponentially in time with a decay constant tau(τ) (i.e., the “ring down time”). If a light-absorbing species isintroduced into the cavity, the light will undergo fewer reflectionsbefore it disappears. In other words, CRDS measures the time it takesfor the light to drop to a certain percentage of its original amount.The time change measured may be converted to a concentration.

As the absorption described above involves hundreds to thousands ofpasses of light through the sample, the sensitivity is greatly enhanced.CRDS is capable of measuring species of known absorption cross sectionsby taking the difference between the ring-down decay rate with sample(1/τ) and the background decay rate without sample (1/τ₀) andmultiplying by L/cl_(s):

$\begin{matrix}{\alpha = {{\frac{L}{{cl}_{s}}\left( {\frac{1}{\tau} - \frac{1}{\tau_{0}}} \right)} = {\sigma\; N}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$where α is the absorption coefficient, c is speed of light, L is thecavity length, and l_(s) is the sample path length. The resultingabsorption coefficient, α, can then be divided by the known crosssection of the sample, or sigma (σ), to yield the concentration ornumber density (N).

In some prior art arrangements, phase shift measurements have relied onthe use of quadrature lock-in amplifiers which are not sufficientlyaccurate at the modulation frequencies encountered in this type ofdevice to provide high phase resolution. In other prior artarrangements, a frequency divider chain starting from a single clock andgenerating both in-phase and quadrature reference signals is used.

Additionally, prior art pertaining to detection of signal time decayrelies on high-speed sampling of the decaying signal followed byconversion to a digital signal. This technique requires the use of anexpensive combination of a laser, a photomultiplier, high-speed samplingand digitizing circuitry. In other prior art pertaining to detection ofsignal time decay, each modulation cycle of the light source is dividedinto a finite number of equal consecutive windows followed byintegration of the light signal recorded during each of those windows.These windows may be generated by division from a single frequencysource. Both methods suffer from deficiencies with respect to precision,cost and robustness of the method.

According to embodiments of the invention, a cavity ring down system canbe optimized to precisely measure trace gases in an air sample by usingtime sampling detection and multiple-sample averaging resulting in ahigh signal-to-noise ratio. In one embodiment, a cavity ring down systemis programmed to measure the rise time and the fall time of the lightlevel in an optical cavity. These values are combined to form a ratio(see Equation 2) that can be used to find the decay time constant (seeEquation 3). More specifically, the cavity ring down system isprogrammed to integrate a plurality of sample portions during a risetime and a plurality of sample portions during a fall time (in alternateintervals). In one implementation, a series of rise time data and aseries of fall time data are collected in alternate intervals. Eachintegrated series gives an indication of the characteristic timeconstant (τ₀) of the cavity that is used to generate a reference timeconstant (see Equation 1). When the rise time measurement and fall timemeasurement (or successive pairs of measurements) are added (i.e., risetime plus fall time), the sum represents the full-scale signal andprovides an indication of the intensity of the light source that isneeded to most accurately calculate the time constant (τ₀) (see Equation1). Additionally, the absolute difference (i.e., rise time minus falltime) between the rise time measurement and the fall time measurementcan also be calculated and divided by the sum (full-scale signal). Whenthe ratio, i.e., (the rise time minus the fall time) is divided by (therise time plus the fall time), represented by the formula:

$\begin{matrix}{{Ratio} = \frac{{{rise}\mspace{14mu}{time}} - {{fall}\mspace{14mu}{time}}}{{{{rise}\mspace{14mu}{time}} + {{fall}\mspace{14mu}{time}}}\;}} & {{Equation}\mspace{14mu} 2}\end{matrix}$this Ratio along with the period (P), the start time of sampling (t₁)and the end time of sampling (t₂) can be input into the followingequation to reiteratively solve for the decay time constant (τ):

$\begin{matrix}{{Ratio} = {1 + {2\left( {1 - \frac{{\mathbb{e}}^{({{{- P}/2}\;\tau})}}{1 + {\mathbb{e}}^{({{{- P}/2}\;\tau})}}} \right)\frac{\tau}{\left( {t_{2} - t_{1}} \right)}\left( {{\mathbb{e}}^{({{- t_{2}}/\tau})} - {\mathbb{e}}^{({{- t_{1}}/\tau})}} \right)}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$The decay time constant (τ) is assumed to be the same for the rise time.The decay time constant (τ) is used to calculate the decay rate (k) asthe inverse of the decay time constant (τ) by the following equation:

$\begin{matrix}{{Rate} = {k\frac{1}{\tau}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

$\begin{matrix}{\alpha = {\frac{1}{c}\left( {k - k_{0}} \right)}} & {{Equation}\mspace{14mu} 5}\end{matrix}$where c is the speed of light, k is the rate obtained in Equation 3(with sample) and k₀ is the rate obtained when the cell is empty (nosample). The absorption coefficient can then be used to calculate thenumber density (N) by the following formula:

$\begin{matrix}{N = \frac{\alpha}{\sigma}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

where N is the number density and σ is the cross section of theabsorbing or scattering species. The concentration in parts per millionor parts per billion may be obtained by dividing the number density bythe total number density (N_(T)) corrected for pressure (P) andtemperature (T) where k is the Boltzmann constant.

$\begin{matrix}{N_{T} = \frac{P}{kT}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

$\begin{matrix}{{{Concentration}({ppb})} = {\frac{N}{N_{T}} \times 10^{- 9}}} & {{Equation}\mspace{14mu} 8}\end{matrix}$

In an alternative embodiment, a cavity ring down system is programmed tomeasure the total signal intensity (I_(o)) and one of the rise time orfall time to obtain a rate equation which can be used to solve for therate of the sample (or no sample) (see Equation 4; Equations 5-8). Moreparticularly, I_(o) may be obtained by taking a measurement while thelight source is ON and taking a measurement while the light source isOFF The difference between these two measurements is I_(o). Thesemeasurements are taken after the decay and rise of light in the lightcavity occur whereas the rise and fall times are taken immediately afterthe light is turned on or off. Either the integrated decay rate or theintegrated rise rate can then be divided by I_(o) to obtain the rateequation which can subsequently be used to obtain a rate equation whichcan be used to solve for the rate of the sample (or no sample). Eitherthe decay rate or the rise rate can be calculated independently from thefollowing equations:

$\begin{matrix}{{{\int_{t_{1}}^{t_{2}}{S{\mathbb{d}t}}} = {I_{o}\left( {{\mathbb{e}}^{- {kt}_{2}} - {\mathbb{e}}^{- {kt}_{1}}} \right)}}\ } & {{Equation}\mspace{14mu} 8}\end{matrix}$

$\begin{matrix}{{{\int_{t_{1}}^{t_{2}}{S{\mathbb{d}t}}} = {I_{o}\left( {t_{2} - t_{1} - {\mathbb{e}}^{- {kt}_{2}} + e^{- {kt}_{1}}} \right)}}\ } & {{Equation}\mspace{14mu} 9}\end{matrix}$

FIG. 1 illustrates a schematic of a CRDS according to an embodiment ofthe invention. Principle components of CRDS 100 generally include(upstream to downstream) a light source 102, an optical cavity 104, adetector 106, an integrator 108 and a converter 110. Light source 102with the aid of an associated lens or lenses 102 a may be directedtoward a proximal end 104 a of optical cavity 104 while a distal end 104b of optical cavity 104 may be directed toward detector 106 with the aidof an associated lens 106 a. In one embodiment, a bandpass filter 112 ispositioned between light source 102 and proximal end 104 a of opticalcavity 104. In an alternative embodiment, bandpass filter 112 ispositioned between distal end 104 b of optical cavity 104 and detector106. Detector 106 may be in electrical communication with integrator108, and integrator 108 may be in electrical communication withconverter 110. All components (i.e., light source 102, optical cavity104, detector 106, integrator 108 and converter 110) may be controlledand/or driven by a computer 114. A quartz crystal oscillator 114 aprovides a stable and accurate timing source for computer 114instruction stepping and timing and, in turn, for the integrationmeasurement interval (explained in more detail below).

In one embodiment, light source 102 may be a non-coherent light sourcesuch as a light-emitting diode (LED). Other examples of light sourcesinclude, but are not limited to, a laser, a blackbody radiator, aflashlamp discharge or other gas discharge. In embodiments in whichlight source 102 is an LED, the LED color and bandpass filter color areselected to provide light in a preferred narrow spectral bandwidth, forexample, between about four-hundred and two (402) nanometers to aboutfour-hundred and twelve (412) nanometers.

In one embodiment, optical cavity 104 may include two highly reflectiveplano/concave mirrors 116 a and 116 b situated internally at each endtherein (i.e., at proximal end 104 a and distal end 104 b of opticalcavity 104). Each mirror 116 a, 116 b may have a diameter ofapproximately one inch (2.54 centimeters). In some embodiments, opticalcavity 104 may have a cylindrical shape and may be, for example, betweenabout 0.25 inches (0.635 centimeters) to about 1.50 inches (3.81centimeters) in diameter, preferably about one inch (2.54 centimeters)in diameter, i.e., approximately close to the effective diameter of eachmirror 116 a, 116 b. The distance between mirrors 116 a and 116 b maybe, for example, between about five (5) inches (12.7 centimeters) andabout fifty (50) inches (127 centimeters). According to one embodiment,a sample inlet 118 is in fluid communication with (or coupled to)optical cavity 104, and, similarly, a pump inlet 120 is also in fluidcommunication with (or coupled to) optical cavity 104. During operationof CRDS 100, a sample may be introduced into optical cavity 104 viasample inlet 118 and removed from optical cavity 104 via pump inlet 120.

In one embodiment, detector 106 with the aid of lens 106 a (i.e.,proximate to distal end 104 b of optical cavity 104) functions tocollect photons emitting from optical cavity 104 continuously or duringpredetermined time intervals (explained in more detail below). Detector104 may be, for example, a phototube (PT), a photomultiplier tube (PMT),or an avalanche photodiode (APD). Integrator 108 (in electricalcommunication with detector 106) collects a current sample from detector106 while converter 110 (in electrical communication with integrator108) measures output voltage from integrator 108.

According to one method, computer 114 drives light source 102 (arrow 122a), e.g., LED 102, via an amplified buffer 122 by generating a squarewave input current which results in LED 102 being repeatedly turned ONand OFF. The amplified buffer 122 may use a constant current source tostabilize the output light level of the LED 102. The period of theresultant modulated current is chosen to be approximately 1/(4*τ) whereτ is a time in microseconds. For example, if τ is two (2) microseconds,then the LED drive period would be eight (8) microseconds and thefrequency would be nominally one-hundred and twenty-five (125) kilohertz(KHz). In another example, if τ is twenty (20) microseconds, then theLED drive period would be eighty (80) microseconds and the frequencywould be nominally twelve and one-half (12.5) KHz. Pulsed lightemanating from LED 102 then illuminates optical cavity 104. The lightlevel in optical cavity 104 builds up, i.e., rises, while LED 102 is ONand then decays, i.e., falls, while LED 102 is OFF.

The light escaping from distal end 104 b of optical cavity 104 isfocused on to detector 106 with the aid of lens 106 a which in turnconverts the photons from the light into electrons. Detector 106collects the photons emitted from optical cavity 104 only when gated(i.e., driven ON by an amplified buffer 124). There are two separatemeasurements made, one during the “ring up” or rise time portion of theresonant cavity cycle (LED 102 ON) and the other during the “ring down”or fall time portion of the resonant cavity cycle (LED 102 OFF). Thesample time signal output (arrow 126) from computer 114 to detector 106defines this gated detection time (see FIGS. 2-3). This process gives asmall current sample which is collected in integrator 108. This processis repeated over, for example, ten-thousand to one hundred thousandsample readings (i.e., for about 0.1 to 1 second) which in turn createsa significant output voltage at integrator 108 (see FIGS. 4-5). Theoutput voltage is then measured by converter 110, which may be, forexample, a high-resolution analog-to-digital converter. After the end ofthe previous measurement cycle and before the beginning of the nextmeasurement cycle, the integrator 108 may be reset (arrow 128) bycomputer 114 and the initial output voltage of integrator 108 may bemeasured by converter 110, i.e., the initial output voltage ofintegrator 108 is measured between cycles. Measuring the initial outputvoltage of the integrator is more accurate than assuming the resetoutput voltage is “zero”. The difference between the final outputvoltage and the initial output voltage, e.g., the Rise Voltage of FIG. 4is the measure of charge (photons) collected during the sampling of the“rise” time over the measurement interval. This process is repeated forthe “fall time” or “ring down” portion of the resonant cavity cycle,i.e., when LED 102 is OFF (see FIG. 5). The “rise” time signal orring-up (i.e., photons captured when LED 102 is ON) and the “fall” timesignal or ring-down (i.e., photons captured when LED 102 is OFF) areconsequently measured and may be used to calculate a ratio, moreparticularly, the ratio of the difference between the rise time and thefall time (i.e., rise time minus fall time) divided by the sum of therise time and the fall time (i.e., rise time plus fall time),represented by the following formula:

$\begin{matrix}{{Ratio} = \frac{{{rise}\mspace{14mu}{time}} - {{fall}\mspace{14mu}{time}}}{{{rise}\mspace{14mu}{time}} + {{fall}\mspace{14mu}{time}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$This ratio may be used to calculate the decay rate (k) represented byEquation 2. This ratio may also be the average of many measurements.

After measurement of the output voltage of the integrator 110, computer114 receives the given measurement (arrow 130) and generates a reading.Such reading may be used to calculate the decay time constant (τ₀) of anempty optical cavity 104 and changes in the decay time (τ) caused byaddition of an absorbing gas or scattering particles to the gas sample.The decay time constant (τ₀) of an empty optical cavity 104 and changesin the decay time (τ) are then used as a reference to calculateconcentration of an absorbing gas (explained in more detail below). Theconcentration of the absorbing gas may be displayed on user interface132 or sent to an external computer before or after processing using theexternal data link (arrow 134). Computer 114 also serves as theregulator and/or controller for the various functions while the CRDS 100is in operation. For example, computer 114, which in some embodimentsmay be a single microchip, controls the square-wave drive of the lightsource 102 (arrow 102 a), the sampling width, position and the intervalof the measurement time (arrow 126) of detector 106, and resetting(arrow 128) of the integrator 108. Computer 114 may also serve tomeasure and control the environment (box 136), i.e., computer 114 canmeasure the atmospheric pressure and temperatures within the unit,and/or, stabilize the temperature of optical cavity 104 and electronics(i.e., making the quartz crystal oscillator 114 a more stable) bycontrolling heaters to keep the temperature constant. Computer 114 mayalso serve to calculate the mathematical algorithms necessary tocorrelate the readings with the quantity of gas detected and/ormeasured.

FIG. 2 is a graphical representation of a “rise” time signal measurementgenerated from a CRDS during a predetermined time period. FIG. 2illustrates the sample time signal in relation to the cavity lightlevel, the LED drive and the integrator input (i.e., the gated photongenerated current from the detector). More particularly, FIG. 2illustrates the cavity light level as a function of the LED being turnedON and OFF. Referring to the LED Drive, the crest of the square waverepresents when the LED is ON while the valley of the square waverepresents when the LED is OFF. For the “rise” time signal measurement,a sample reading is taken during the time period when the LED is ON,i.e., the “ring up” or rise time.

FIG. 3 is a graphical representation of a “fall” time signal measurementgenerated from a CRDS during a predetermined time period. FIG. 3illustrates the sample time signal in relation to the cavity lightlevel, the LED drive and the integrator input (i.e., the gated photongenerated current from the detector). More particularly, FIG. 3illustrates the cavity light level as a function of the LED being turnedON and OFF. Referring to the LED Drive, the crest of the square waverepresents when the LED is ON while the valley of the square waverepresents when the LED is OFF. For the “fall” time signal measurement,a sample reading is taken during the time period when the LED is OFF,i.e., the “ring down” or fall time.

FIG. 4 is a graphical representation of an example rise time measurementinterval. It depicts a time interval of between 0.1 and 1.0 seconds. Itshows the resultant integrator output voltage, or “rise” voltage, whichis the measurement of the rise time signal component. It also shows anexample of the integrator reset time and the ADC conversion times justbefore the start of integration and just after the completion of theintegration.

FIG. 5 is a graphical representation of an example fall time measurementinterval. It depicts a time interval of between 0.1 and 1.0 seconds. Itshows the resultant integrator output voltage, or “fall” voltage, whichis the measurement of the fall time signal component. It also shows anexample of the integrator reset time and the ADC conversion times justbefore the start of integration and just after the completion of theintegration. The “fall” voltage represented in FIG. 5 is less than thecorresponding “rise” voltage represented in FIG. 4.

The addition of an absorbing gas (such as certain gases found inpolluted air) causes the rise and fall times to decrease. These changesin the rise and fall times can be detected and measured and then used tocalculate the amount of an absorbing gas present by comparison with the“no sample” reference data (explained previously). Thus, according to anembodiment of a method of the invention, the above process may beperformed while a sample is introduced via sample inlet 118 and pumpedout via pump outlet 120 to obtain data correlating to the introducedsample. In one embodiment, the sample may be introduced continuously.The CRDS 100 may detect absorbing species in gas samples with anyparticles present filtered out or, alternatively, CRDS 100 may detectparticles if any absorbing gases such as NO₂ are removed, i.e., filteredout. Examples of absorbing gases that may be measured and quantifiedaccording to embodiments of the invention include, but are not limitedto, nitrogen dioxide, nitrogen trioxide, fluorine, chlorine, bromine,iodine, ozone, sulfur dioxide, chlorine dioxide, HO₂ radicals, hydroxyradicals, and hydrocarbons, including aldehydes and aromatic species.

In contrast to prior art methods which look at decay rate only, themethods according to embodiments of the invention take into account boththe rise and fall rates of the light emitted from a light source togenerate a more precise reading of absorbing gases within a sample. Suchmethod may be referred to as “time domain sampling.” A key element totime domain sampling is the gated light detection performed by thedetector, which allows photons to be detected and integrated withoutincluding wide bandwidth electronic noise associated with narrow timedomain samples. Assuming photon noise dominates, the precision of ameasurement may be doubled by increasing the integration time by afactor of four in view of that the photon noise level is the square rootof the number of photons collected.

Representatively, a sample of 1000 collected photons results in a noiselevel of 3%, i.e., the square root of 1000 is approximately 30 which isthen divided by 1000. On the other hand, a sample of 1,000,000 collectedphotons results in a noise level of 0.01%, i.e., the square root of1,000,000 equals 1000 which is then divided by 1,000,000. Thus,according to an embodiment of the invention, integration of many samplesgives a high photon count with a low noise, i.e., low signal to noiseratio, resulting in better precision and stability.

Advantageously, embodiments of devices and methods of the invention lenditself to higher precision stability via the more precise referencemeasurements (explained previously), temperature control, atmosphericpressure measurement, and a minimum number of analog and digital parts,e.g., low noise operational amplifier, a stable integrating capacitor, ahigh resolution delta-sigma ADC, the LED and the phototube. Moreover,embodiments of devices and methods of the invention lend itself to aminimum cost and minimum number of components for its performancecapability. For example, buffers to drive the LED and the phototube, anintegrator (an op amp, summing capacitor, and reset circuitry), a highresolution ADC, a display (an LCD driver and LCD) and an externalinterface can be controlled by a single chip microcontroller.Furthermore, embodiments of devices of the invention are flexiblebecause many variables are controlled by the software, e.g., the cycleperiod, the LED ON and OFF time, the sample time, the reset time and theintegration time. This lends itself to easy adjustability to, forexample, changes in the optical cavity's length or mirror reflectioncoefficients can be accommodated by simply changing the measurementtiming through changes in the software when needed. In some embodiments,the software can automatically optimize measurements.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention is not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

1. A method of measuring the presence of a gas in an air sample using acavity ring down system, comprising: when no sample is introduced intothe system: measuring an output voltage from a plurality of currentsresulting from light introduced into an optical cavity when a lightsource is ON during a predetermined interval of a rise time of the lightwherein the resulting measurement is a reference rise time measurement;measuring an output voltage from a plurality of currents resulting fromlight introduced into the optical cavity when the light source is OFFduring a predetermined interval of a fall time of the light wherein theresulting measurement is a reference fall time measurement; measuring anoutput voltage from a plurality of currents resulting from lightintroduced into the optical cavity when a light source is ON during therise time of the light wherein the resulting measurement is a referencesteady state rise time measurement; and measuring an output voltage froma plurality of currents resulting from light introduced into the opticalcavity when a light source is OFF during the fall time of the lightwherein the resulting measurement is a reference steady state fall timemeasurement; wherein the reference rise time measurement, the referencefall time measurement, the reference steady state rise time measurement,and the reference steady state fall time measurement are used tocalculate the amount of gas in the air sample by a computer.
 2. Themethod of claim 1, further comprising: when an air sample is introducedinto the system: measuring an output voltage from a plurality ofcurrents resulting from light introduced into an optical cavity when alight source is ON during a predetermined interval of a rise time of thelight wherein the resulting measurement is a sample rise timemeasurement; measuring an output voltage from a plurality of currentsresulting from light introduced into the optical cavity when the lightsource is OFF during a predetermined interval of a fall time of thelight wherein the resulting measurement is a sample fall timemeasurement; measuring an output voltage from a plurality of currentsresulting from light introduced into the optical cavity when a lightsource is ON during the rise time of the light wherein the resultingmeasurement is a sample steady state rise time measurement; andmeasuring an output voltage from a plurality of currents resulting fromlight introduced into the optical cavity when a light source is OFFduring the fall time of the light wherein the resulting measurement is asample steady state fall time measurement.
 3. The method of claim 2,wherein the calculated amount of gas in the air sample is refined byincluding, the sample fall time measurement, the sample steady staterise time measurement and the sample steady state fall time measurement.4. The method of claim 3 wherein each output voltage is collectedbetween 10,000 and 100,000 times.
 5. The method of claim 1 wherein thelight source is a light-emitting diode or laser.
 6. The method of claim1 wherein the absorbing gas is at least one of nitrogen dioxide,nitrogen trioxide, nitrous oxide, fluorine, chlorine, bromine, iodine,ozone, sulfur dioxide, chlorine dioxide, HO₂ radicals, OH radicals,formaldehyde, aldehydes, hydrocarbons, or an aromatic species.
 7. Themethod of claim 1 wherein the combination of absorption and scatteringof particles is measured by the cavity ring down system.